A known process for ultrahigh purity silicon production involves the upgrading of metallurgical grade silicon feedstock to an ultrahigh purity silicon powder product. The process involves three distinct components, i.e., a hydrogenation subsection, a redistribution reactor and column subsection, and a silane pyrolysis and silicon powder consolidation subsection. The hydrogenation subsection involves the reaction of the metallurgical grade feedstock with recycle silicon tetrachloride and hydrogen to produce an intermediate chlorosilane product; during such procedure heavy waste materials including many of the metallurgical impurities are removed as a sludge waste stream. The trichlorosilane containing feedstock is then passed to a middle reactor and column section whereby a combination of reactors containing resin catalyst and distillation columns are utilized to upgrade the chlorosilane to an ultrahigh purity silane product and a recycle tetrachloride stream. The final subsection of the process utilizes the ultrahigh purity silane to produce the ultrahigh purity silicon product material, e.g., the silane feedstock material can be pyrolyzed utilizing homogeneous decomposition reaction of the silane in a free space reactor. Such a reactor involves the turbulent injection of the silane feedstock with suitable applied heat so that the silane is decomposed to hydrogen and silicon powder. The hydrogen is removed from the reactor and recycled back to the hydrogenation section and the powder can then be consolidated into a readily handleable form.
A prior art free-space reactor apparatus is described in U.S. patent application Ser. No. 902,562 filed May 3, 1978 which is incorporated herein by reference. The above-noted prior art reactor involves a cylindrical vessel and an axially centered nozzle for introduction of silane gas as shown in FIG. 1 of the present patent application. The prior art reactor comprises a vertical cylindrical vessel with an inner quartz liner and a circumferential array of uniformly spaced induction coils on the outside for heating purposes. The arrangement also includes a suitable nozzle placed on the central axis of the reactor to introduce the silane, and suitable reactor heads for necessary stream connections. The reactor head includes a water cooled jacket. Another feature is a pneumatically operated scraper which used to remove soft silicon powder from the wall.
Early testing with the prior art FIG. 1 apparatus indicated the basic operational feasibility for the free space reactor and further work lead to the discovery of several problems. Temperature instrumentation indicated an essentially parabolic temperature profile along the axial length of the reactor as shown in FIG. 2 of the present application. This non-uniform temperature profile resulted in the formation of considerable hard silicon deposits on the reactor wall at the silane inlet end of the reactor. These silicon deposits could not be readily removed with the scraper arrangement and often led to quartz liner failure. Additionally, examination of the reactor following experimentation indicated that silane gas had migrated between the liner and reactor walls and caused corrosive attack on the reactor wall.
Analysis of the experiments indicated unsuitability of the above-noted prior art free space reactor and an improved heating method and apparatus arrangement is provided which is the subject of the present invention.